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Old 18th January 2009, 08:07 PM   #41
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From the above measurement it shows that the first dipole dip is about 2.3dB at 1.2kHz.

If your tweeter can not be crossed that low then I would expect the dip is audible, which can impose a small amount of colouration to the sound, given that our ears are highly sensitive between 1kHz - 4kHz.

The baffle needs to be narrowed down (nearly halved) to push this dipole dip to about 2kHz, so that it can be dealt with with the crossover. From my memory, the NaO has a baffle width of 28cm and the first dipole dip is around 2kHz.

However, in that case, the first dipole peak is much higher, and you can expect the dipole rolloff comes in much early. The P13 (it is a 5" from my memory) can easily run out of excursion and I am not sure if you can cross it anywhere lower than 300Hz-350Hz for a reasonable sound pressure capability.

That is why the Orion uses a 8" woofer and the NaO uses twin 6"5 woofers in a MTM. The larger cone area is needed for the dipole rolloff. The larger drivers are also more directive at higher frequencies, therefore reducing the dip at 2kHz. They are low-passed at about 120Hz - 150Hz.

If the P13 is crossed over above 300Hz, the woofers may not work in a H-frame or U-frame because of the 1/4 resonance. The resonance for a sealed box is twice higher at 1/2 wavelength instead of 1/4. But in practice, I have my U-frame heavily damped and I could not detect any resonances. I am crossing my woofers at 175Hz and it seems the bass is very clean.
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Old 18th January 2009, 08:32 PM   #42
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Just a thought with regards to your baffle width.

I have had this idea for a long time but have not had an opportunity to try it.

How about adding sound absorbing materials on your baffle? For example, wool batts / blankets or even open ceil foam, attaching them on both sides of the panel just about an inch away from the driver edge and extending right beyond the baffle edge for up to 2 inches. Note that sound absorbing is only effective when the sound absorbing materials are as thick as about 1/4th of a wave length. So for a 1kHz signal, the sound needs to travel 34cm / 4 = 8cm through the damping material in order for the sound to be sufficiently damped.

Such damping should almost completely eliminate the dipole dip, allowing you to keep your existing wide baffle and use the small P13 to do the work without suffering the loss of efficiency. One headache I have with dipole speakers are the severe diffraction effects, which also seems to be visible from your measurements. Using the above method can possibly eliminate the diffraction effects.

As for asthetics, I was thinking about getting a upholsterer to make something to wrap around the damping material / speaker panel, with nice looking fabric. It can be made to look stunning.

Regards,
Bill
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Old 18th January 2009, 10:46 PM   #43
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Default S7

Sorry to not updating as I have been very lazy and the weather was so nice yesterday


I solved the problem by redesigning the baffle. P13 and tweeter mounted on small, tall baffle and the woofers are separate. It looks really pretty to my surprise, no more huge monolith.

Click the image to open in full size.

Most importantly, I did outside measurement and they reflect the theoretical dipole models. Off-axis response is smooth. The pic below is raw, unequalised response (valid above 200Hz or so). The notch is very easily implemented.

Click the image to open in full size.

How good does it sound? Well, I've packed up even before my usual "let's hear them for a week" . Either this is my "last speaker" or maybe the curse of OB gods will haunt me to $pend and build an Orion.

I'll try to post a picture soon.
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Old 18th January 2009, 11:01 PM   #44
tinitus is offline tinitus  Europe
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Just yesterday I thought of this fellow, all drivers the same
Could be the one of new underhung TB 4"

It wasnt thought as a dipole, but I suppose it would work nicely as dipole too, maybe with some kind of woofer module
Crossover is a quite simle 2.5way...which means 6 drivers in all will play midbass, and ought to result in low distortion
One driver is solely "tweeter", crossed somewhere lowish
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Old 18th January 2009, 11:22 PM   #45
tinitus is offline tinitus  Europe
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A low distortion sub module could maybe be like this push-pull "OB"
Driver polarity is reversed so its dipole allright, but not sure how it works really
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Old 19th January 2009, 09:07 AM   #46
Rudolf is offline Rudolf  Germany
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gainphile,

from your drawing it looks like your tweeter now is almost equidistant from the top and the sides of the baffle. If thatīs the case: Is it intentional or by chance? In the first case you need to equalise a dipole peak for the tweeter too. In second case I would recommend to cut the baffle just above the tweeter to make the edge radius around the tweeter as "divergent"(?) as possible. Perhaps you should simulate it in EDGE.
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Old 19th January 2009, 10:19 AM   #47
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Product shot... finally

Click the image to open in full size.


Rudolph, that's a great insight. I honestly did not simulate the tweeters as this one uses a low-end one, I have not decided on what tweeters to be used. In saying that I have no complaint about high-end smoothness and extension.

Tinitus,
Yes as soon as I mounted the mid and tweeter on that skinny baffle with pretty pine grain, the immediate thought was... wow these have great WAF I wonder if 4 or 5 small woofers will do. We'll save that for S8 okay ?

Finally my insights to this point:

1. The difficult part of OB is not bass (surprisingly) but midrange. OB midrange will have peaks which needs to be corrected. Midrange should use the lower part of dipole peak, not the upper part where freq. response is not smooth, although low-freq. rolloff is avoided.

2. Big baffles to extend low-end frequency just doesn't cut it for the midrange. It will suffer.

3. OB can be simulated using good software such as John's ABC Dipole program. I know that SL thinks otherwise.
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Old 19th January 2009, 03:59 PM   #48
Rudolf is offline Rudolf  Germany
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Those look really great, gainphile.

I would rate their WAF much higher than the Plutos in the back. And I fully support insights 1-3. Although I donīt see, why SL would oppose Nr. 3 more than JohnK would.

If those woofers would be in a shallow H frame (total depth 20 cm), giving them a little bit more depth/volume, the nice aesthetic balance between mid-high slenderness and down-to-earth bass could be even more convincing IMHO.

Did I say I really like that combination?
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Old 20th January 2009, 11:33 AM   #49
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would it be possible to hide the woffers perpendicular on the back of the baffles? would it still sound right? you probabily can give it a fast try... pardon me if it is a dummy question...
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Old 20th January 2009, 12:38 PM   #50
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and the second dummy question is...

is the tweeter working dipole this way? or should you be considering to add a another on the back, like in the orion++?
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